The present disclosure relates to systems and methods for processing screen rejects in sulfite pulping.
Chemical paper pulping is a process in which wood chips are initially chemically processed in a digester in order to reduce the lignin content, prior to bleaching. After the initial digestion step, undercooked wood chips are separated from the resulting pulp fibers using a screening system (e.g. also known as a knotter), which separates a reject portion from an accept portion. The reject portion typically includes residual wood chips that have not been sufficiently digested, such as knotted or oversized chips, which are commonly referred to as “screen rejects” or simply “rejects”. While the accept portion is suitable for further processing to produce pulp/paper, the reject portion is removed, thus resulting in a loss of material and a reduction in yield. The screen rejects thus represent a material loss within the chemical pulping process and it is generally desirable to reprocess or reuse the screen rejects.
In the Kraft process, which is a sulfate chemical process, the Kraft screen rejects have a low lignin content and low level of lignin condensation, rendering Kraft screen rejects amenable to a wide range of reprocessing methods. For example, as taught in U.S. Pat. No. 5,672,245, Kraft screen rejects, having been separated from the accept portion, may be returned to the chip chute for further digestion in the digester along with fresh wood chips. Similarly, as taught in PCT Patent Application No. PCT/SE2010/050181, Kraft screen rejects may be recirculated to the digester using a chopper pump.
Another form of chemical pulping is the sulfite pulping process, which due to the different digestion chemistry employed, does not produce screen rejects that are suitable for reprocessing. In particular, sulfite rejects differ from Kraft rejects in both hardness (lignin content) and lignin structures. The lignin content of sulfite rejects can be high relative to the hardness of rejects from the Kraft process. Furthermore, the lignin present in sulfite rejects is highly condensed due to the acid-induced lignin condensation in the digestion step. For these reasons, known reject processing and reclamation methods, such as those employed in mechanical pulping, or in the Kraft process, have not been suitable for processing sulfite screen rejects.
Presently, due to their incompatibility with screen processing methods tailored for the Kraft process, sulfite rejects are often disposed by burning them in a boiler, in order to extract residual energy. Unfortunately, the high sulfur content of the sulfite rejects makes this option less favorable, partly because of the potential for corrosion in the boiler, and also because of air pollution caused by the formation of sulfur dioxide. Alternatively, sulfite rejects may be removed and sent to a landfill. This approach is problematic due to the high landfill fee and in the potential environmental hazard.